1. Field of the Invention
The present invention relates to a sensing device for sensing a substance to be sensed contained in a sample solution based on a natural frequency of a piezoelectric resonator such as a quartz-crystal resonator.
2. Description of the Related Art
As a sensing device for sensing and measuring a trace substance contained in a sample solution, there has been known one which uses a quartz-crystal sensor. As this type of sensing device, one that detects a substance to be sensed while letting a sample solution flow, as disclosed in Patent Document 1, has been known. A quartz-crystal piece 10 of a quartz-crystal sensor of the sensing device is placed on a wiring board 13 so as to block a hole portion 13a of the wiring board 13 as shown in FIG. 11, and is pressed by a quartz-crystal pressing member 14 when an annular pressing portion 14a of the member abuts thereon. Further, in a state where the quartz-crystal sensor is mounted on a bottom portion of a case 16, a reaction channel 17 is formed to surround an area facing a front surface of the quartz-crystal sensor, and at this time, a substance to be sensed in a sample solution is absorbed in an absorption layer 12 on the quartz-crystal piece 10. A buffer solution is flowed through the reaction channel 17 before letting the sample solution flow through the channel, and by measuring a variation in frequency in the quartz-crystal piece 10 at this time, a concentration of the substance to be sensed in the sample solution is measured.
Meanwhile, the present inventor has been studying a quartz-crystal sensor of twin sensor type. This quartz-crystal sensor includes two excitation electrodes (a detection electrode and a reference electrode) on one surface side of a quartz-crystal piece to reduce an influence of an external factor of measurement environment such as a temperature, for instance, in which an absorption layer formed of a biological substance or the like is formed on a front surface of the detection electrode. Note that by corresponding to the excitation electrodes, excitation electrodes are provided also on a rear surface side of the quartz-crystal piece, thereby forming an oscillation area for detection and an oscillation area for reference. Accordingly, since only the detection electrode is affected by a mass change caused by an absorption of a substance to be sensed, by subtracting an oscillation frequency taken out from the reference electrode from an oscillation frequency taken out from the detection electrode, it is possible to obtain a highly accurate measured result from which the external factor is removed.
Meanwhile, when the measurement is conducted under a liquid phase atmosphere, a sensitivity of quartz-crystal sensor is deteriorated due to a viscosity of a liquid. Accordingly, in order to improve the sensitivity, it is desirable that the detection electrode and the reference electrode are designed to be large in size.
However, since the detection electrode is affected by an oscillation energy from the reference electrode, in order to reduce an influence of noise due to the oscillation on the detection result, it is preferable to increase a space between these electrodes. To ensure the space, the size of each of the electrodes has to be reduced, which also reduces a forming area of the absorption layer, resulting in a problem that the measurement sensitivity is deteriorated.
[Patent Document 1] Japanese Patent Application Laid-open No. 2008-58086 (paragraph [0014], FIG. 11 and FIG. 13)